In an exhaust gas purifying system, which is provided with an air/fuel ratio sensor upstream of a catalyst disposed on the exhaust pipe of the internal combustion engine so that the output of the air/fuel ratio sensor may approach a target air/fuel ratio. Moreover, another air/fuel ratio sensor is further disposed downstream of the catalyst so that the target air/fuel ratio upstream of the catalyst may be corrected on the basis of the output of that downstream air/fuel ratio sensor.
However, in this system the output characteristics are varied even at the same air/fuel ratio by the temperature change of a solid electrolyte element (or a sensor element) of the air/fuel ratio sensor. In JP-A-9-127035, for example, therefore, the detection precision is improved by controlling the electric current of a heater for heating the sensor element thereby to make the element temperature of the air/fuel ratio sensor constant. In U.S. Pat. No. 5,263,358, moreover, the detection precision is improved by correcting the sensor output characteristics according to the sensor element temperature of the air/fuel ratio sensor. These technologies can improve the detection precision to the air/fuel ratio but not the detection precision (or reaction) to a specific gas.